1. Field of the Invention
The present invention relates to an LED device and related method, and more particularly, to an LED device with a simultaneous open and short detection function and related method.
2. Description of the Prior Art
Light emitting diodes (LEDs) used as light sources has become popular in recent years. For example, cold cathode fluorescent lamps (CCFLs) are conventionally used as a light source in a backlight module of a liquid crystal display. However, LEDs have gradually replaced CCFLs as the light source of the backlight module due to continuously rising luminous efficiency and decreasing cost.
In an LED driving circuit of the prior art, if LED open occurs on an LED string, since a corresponding output channel of the LED driving circuit is floating, the LED driving circuit would have electric leakage, which deteriorates conversion efficiency of the circuit or results in abnormal operation of a voltage conversion loop. Besides, if LED short occurs on an LED string, i.e. cross voltages of some LEDs are zero, headroom voltages of current driving elements would be raised correspondingly, which results in higher power consumption of the current driving elements and deteriorates the conversion efficiency of the circuit as well. Therefore, the LED driving circuit should have LED open and LED short detection mechanism.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of an LED driving circuit 10 according to the prior art. The LED driving circuit 10 is utilized for driving an LED module 11. As shown in FIG. 1, the LED module 11 includes parallel-connected LED strings C1˜Cm, and each LED string further includes a plurality of series-connected LEDs. The LED driving circuit 10 includes a voltage converter 12, a current driving unit 13 and a loop control unit 14. The voltage converter 12 is utilized for converting an input voltage V1 to an output voltage V2 according to a voltage control signal VCTRL so as to drive the LED module 11. The current driving unit 13 is utilized for sinking fixed driving currents Id1˜Idm from the LED module 11. The loop control unit 14 controls voltage conversion of the voltage converter 12 according to voltage differences between negative electrode voltages VHR1˜VHRm of the LED strings C1˜Cm and a default reference voltage VREF, for stabilizing a voltage level of the output voltage V2.
Moreover, the loop control unit 14 further includes a voltage selector 142, an error amplifier 144 and a conversion controller 146. The voltage selector 142 is coupled to the LED strings C1˜Cm, and is utilized for selecting a lowest voltage of the negative electrode voltages VHR1˜VHRm as a feedback voltage VFB. The error amplifier 144 is coupled to the voltage selector 142 and the reference voltage VREF, and is utilized for generating an error voltage signal VERR according to voltage difference between the feedback voltage VFB and the reference voltage VREF. The conversion controller 146 is coupled to the error amplifier 144 and the voltage converter 12, and is utilized for generating a voltage control signal VCTRL according to the error voltage signal VERR.
Therefore, through the loop control unit 14, the LED driving circuit 10 can lock the negative electrode voltages VHR1˜VHRm of the LED strings C1˜Cm, i.e. the headroom voltages of the current driving elements, and the output voltage V2 of the voltage converter 12 within a sensible range.
In this case, the LED driving circuit 10 further includes an open detector 15 and a short detector 16, which are utilized for performing LED open detection and LED short detection on the LED strings C1˜Cm, respectively. Since the headroom voltages of the current driving elements would be pulled to a low voltage level when the LED strings C1˜Cm have LED open, the open detector 15 can thus determine the LED open occurring on the LED strings C1˜Cm according to whether the negative electrode voltages VHR1˜VHRm of the LED strings C1˜Cm are lower than a certain low threshold voltage. Of course, the said low threshold voltage cannot be set higher than the headroom voltages of the current driving elements under normal operation for preventing from false LED open detection during the normal operation situations. On the contrary, when the LED strings C1˜Cm have LED short, i.e. cross voltages of some LEDs are zero, the headroom voltages of the current driving elements would rise correspondingly. Thus, the short detector 16 can determine the LED short occurring on the LED strings C1˜Cm according to whether the negative electrode voltages VHR1˜VHRm of the LED strings C1˜Cm are higher than a certain high threshold voltage. Similarly, the said high threshold voltage cannot be set lower than the headroom voltages of the current driving elements under the normal operation for preventing from false short detection during the normal operation situations.
However, the LED driving circuit 10 may erroneously determine the LED short occurring on the LED strings C1˜Cm when simultaneously performing the LED open and short detection on the LED strings C1˜Cm. For example, when the LED string C1 has the LED open, the headroom voltage of the current driving element is pulled to a low voltage level (ex. a ground voltage). Thus, the voltage selector 142 would select the negative electrode voltage VHR1 of the LED string C1 as the feedback voltage VFB, such that the output voltage V2 of the voltage converter 12 is raised. Under this situation, since the cross voltages of the LEDs are fixed, the negative electrode voltages VHR2˜VHRm of the LED strings C2˜Cm would follow the output voltage V2 to rise above the said certain high threshold voltage, which results in false determination of the short detector 16.
In other words, when the LED open and the LED short detection are simultaneously performed on the LED strings, the prior art may have false LED short detection immediately after the LED open is detected on some of the LED strings.